[spiderman1441]

During the precipitation process of cations of group 5 (Ba⁺² ,Sr⁺² ,and Ca⁺² ) , We Add drops of  (NH₄)₂Cro₄(we add it as drpos slowly to decrease the possibility of precipitation of Srcro₄  with barium chromate) with heating (40-60 degrees ) to precipitate barium chromate ( Yellow ppt ) , Then we take the supernanent layer of Sr⁺² and Ca⁺² , and add aq. NH₃ + Ethanol to precipitate stratium chromate , it will precipitate as stratium chromate yellow ppt , but Ca⁺² will not precipitate . we can precipitate calcium by addition of (NH₄)₂C₂O₄ .

The Question is :  why Cacro₄ , don't precipitate with stratium chromate due to the very small different of ksp values between them, i.e  when we added ethanol in aq. amonia to precipitate strantium chromate ,why we didn't expect that calcium chromate may precipitate with stratium chromate specially when we see that both of them have a very small different of ksp values.?


                         

ksp

Barium chromate = 2.2x10^-10

Calcium chromate = 7.1x10^-4

Stratium chromate = 2.2x10^-5

[AWK]

Addition of ethanol differentiates solubilities of chromates. In water-alcohol solution Ksp values are not applicable
(they differ from that of  for water solutions)